In the manufacture of paper or paperboard, it is common to employ large attrition mills to grind wood chips or other fibrous raw materials to produce pulp, or to grind chemically produced wood pulp to enhance its papermaking properties. In both cases, the process is referred to as refining. These attrition mills are normally of the disk type or the conical type (or sometimes a combination of the two), where a rotor surface acts against a stator surface (or in some instances a counter-rotating surface) and causes a reduction in the size or a change in some other desirable physical properties of the material being processed. The working surfaces of these mills usually consist of a stator plate with more or less radial bars and grooves, and a rotor plate of similar form. The material being processed, often fibrous in nature, is captured between a rotor bar edge and the opposing stator or counter-rotating bar edge. It is the compression loading of the fibrous particles which acts to cause a change in the physical properties of the material being processed.
The wear surfaces of these grinding mills (called refiner plates or refiner fillings) are replaceable and may be require replacement at intervals between a few weeks and several months or more. They are usually made of cast steel but may also be fabricated or machined from solid steel blanks. During the normal course of refining of the wood chips or the pulp, it is the wearing down of the bars on the opposing surfaces which eventually leads to the need for replacement.
The most common control parameter in the refining of wood chips or pulp is the applied power. More precisely it is the net applied power that is of significance, since a certain amount of the input shaft horsepower is consumed by viscous frictional losses in the fluid which suspends the process particles (either a vapor or liquid phase). The net applied power is a measure of the amount of energy that is being applied to a given flow of process material and is referred to as the specific energy consumption (often expressed as kilowatt-hours per ton of moisture free material processed).
It is well known in the pulp and paper industry, that specific energy consumption (SEC) is not the only significant parameter that influences the quality characteristics of the material being processed. A second parameter, which reflects the magnitude of the compressive loads applied to the fibrous particles, should also be significant. This second parameter is called refining intensity. There has not previously been any means to directly measure refining intensity, and it is usually inferred by a parameter called specific edge load (SEL). SEL is usually computed by carefully measuring the total length of the stator and rotor bar edges that will cross in a single revolution. The net applied power divided by the product of the total edge length and the rotational speed yields a value for the specific edge load (usually expressed as watt-seconds per meter).
The two-parameter concept of refining has been viewed in a variety of ways. One such view identifies a first parameter as a measure of the number of impacts that act on an average particle, and a second measure as the intensity of the impact that acts on the average particle. However, all such views depend on the measurement of the edge length of the working surface of the filling and take no account of the extent to which material is in fact captured on the available edge length. Other process variables including the condition of the process material, the condition of the bar edge, the angle of intersection of rotor and stator bars and the flow velocity in the filling, all may have significant effects on the amount of process material actually captured on the edges. Indeed, there are many instances in both pilot plant and commercial experience, where a particular pulp processed under identical conditions of SEC and SEL has exhibited significantly different measured physical characteristics.
Refining intensity has long been considered a parameter of interest in low consistency refining of paper pulps using bar equipped beating devices. It is now generally accepted that the refining effect on pulp in any given refiner is largely determined by the amount of refining (the specific energy consumption, or SEC) and the intensity of refining (the specific edge load, or SEL). Even in comparing the effects of different refiners of different size and process flow, these two parameters have proven to be reasonably predictive of pulp characteristics and the resulting paper properties—at least qualitatively if not quantitatively. They are often described as the “amount” and the ‘severity” of refining, respectively.
The calculation methods for the two parameters are simple and they will not be presented here. SEC is arguably a fundamental process variable (energy input per unit mass of moisture free substance). While the energy may be applied more or less efficiently in terms of producing some desired effect, it is conceptually easy to appreciate its potential impact on the refining result. SEL, on the other hand, represents a machine parameter (a function of edge length available and rotational speed rather than a process condition. It is generally presumed to be indicative, at least on a relative basis, of the severity of the stress acting on the fibers in the process. However, it does not account for what may be very large variations in the collection of pulp fibers on bar edges due to such factors as pulp consistency, flow velocities, bar edge sharpness, or degree of refining. In attempting to optimize refiner fillings and operating conditions, it is often not sufficiently predictive to meet the needs of some modern papermaking operations, and it offers no diagnostic help when an unexpected result is realized.
In general, while SEC and SEL are somewhat predictive of the product quality characteristics, a more direct measure of the actual strains applied to the process material would be very useful. It could be used in the diagnosis and control of disk mills, in particular with regard to the design and development of more energy efficient refiner fillings, and with regard to optimizing the operating conditions of the process so as to produce higher quality products.
It has long been recognized that the operating clearance between the rotor and stator will be of significant importance in a disk mill. It is not uncommon in modern commercial chip refining systems to have several refiners equipped with clearance measuring devices. However, the difficulty in maintaining the precision and reliability of the devices, particularly with regard to the zero reference, has made them of limited value in routine diagnosis and control of refiners. Because the bars of the working surface wear continuously and in a very irregular way, and because of the very hostile environment in which they operate, delicate gap measuring instruments are often not reliable.
Nonetheless, operating clearance or gap remains an important operational factor and the present invention takes into account a “delta g” or the change in gap (instead an absolute value for gap) in providing a direct measure for refining intensity.